#include <stdio.h>

unsigned long long Fib(int x)
{
	if(1 == x)
		return 0;
	if(2 == x)
		return 1;
		
	return Fib(x-1) + Fib(x-2);
}

int main()
{
	int n = 0;
	unsigned long long m = 0;
	scanf("%d", &n);
	m = Fib(n);
	printf("%u", m);
	
	return 0;
}

